Unraveling the Catalytic Mechanism of Rutile RuO2 for the Oxygen Reduction Reaction and Oxygen Evolution Reaction in Li-O2 Batteries

Because of the involvement of solid-state discharge product Li2O2. how a catalyst works in nonaqueous lithium oxygen batteries is yet to be determined, although the question has undergone fierce debate. In this work, we take an effective and widely used catalyst, rutile RuO2, as a representative and studied its catalytic mechanism in lithium oxygen batteries via ab initio calculations. For the oxygen reduction reaction (ORR), it is found that rutile RuO2 can provide large adsorption energies toward LiO2 and Li2O2, thus resulting in high initial discharge voltages. Moreover, the normalized degree of unsaturation of surface oxygen is identified as a descriptor for the ORR catalytic activity. For the oxygen evolution reaction (OER), we propose that, in addition to the three-phase interface, the OER may also occur at the two-phase interface of Li2O2/RuO2, where rutile RuO2 provides pathways for the lithium ions while oxygen evolves from the exposed surfaces of Li2O2. Calculation results show that our proposed catalytic scenario is both thermodynamically and kinetically viable. Along with the charge process, the remaining Li2O2 can be attracted to the catalytic surfaces spontaneously, which can effectively preserve the reaction interface.